1. Field of the Invention
The present invention relates to a liquid crystal display device, and, in particular, to a VA mode liquid crystal display device excellent in the viewing angle property and a polymer film etc, to be used in the display device.
2. Description of the Related Art
The application of liquid crystal display devices has been increasing year by year as electric-power saving and space-saving image display devices. The existing liquid crystal display devices have a large defect of having large viewing angle dependency of images. In the recent years, however, a wide viewing angle liquid crystal mode has been put into practical use, which brings about a rapid increase in the demand of liquid crystal display devices even in a market of, for example, televisions with a need for high grade images.
Although VA mode liquid crystal display devices are advantageous in showing high contrast in general compared with devices having other liquid crystal display modes, they suffer from such problem that the contrast and hue largely change depending on the viewing angle.
To solve this problem, optically compensatory films having various optical properties and methods for combining the same have been proposed hitherto. Among all, there is a highly promising method comprising combining an optically compensatory film in which the in-plane retardation (hereinafter referred to as Re) becomes smaller at a shorter wavelength (hereinafter referred to as inverse dispersion) with another optically compensatory film in which the retardation in the thickness direction (hereinafter referred to as Rth) becomes larger at a shorter wavelength (hereinafter referred to as normal dispersion). For example, JP-A-2007-156433 discloses a method wherein a stretched cellulose acylate film is used as an optically compensatory film with inverse dispersion while a film having a polyimide layer formed on a tack is used as an optically compensatory film with normal dispersion. Although use of such a film having a polyimide layer formed on a tack is an effective way for obtaining an optically compensatory film with normal dispersion, it costs high because of the double-layered structure.
To improve weatherability of polymer films such as cellulose acylate films, on the other hand, there has been known a method of adding an additive having an absorption in the ultraviolet wavelength region (hereinafter referred to as a UV absorber). As the UV absorber, a colorless compound showing an absorption band in the ultraviolet region of as long wavelength as possible is preferred. JP-A-2002-47357 discloses a cellulose acylate film containing a compound having a structure wherein an amino group is attached to one end of a butadiene structure and an electron-withdrawing group is added to the other end thereof (hereinafter referred to as an aminobutadiene compound). However, the aminobutadiene compound is liable to be degraded after prolonged photo-irradiation and, therefore, further improvement is still required. Moreover, JP-A-2002-47357 refers nothing to the use of an aminobutadiene compound for controlling the retardation, in particular, controlling the wavelength dependency of the retardation in the thickness direction.
In the case of adding an aminobutadiene compound to a cellulose acetate film, in particular, bleed-out frequently occurs. As a result, there arises a problem that the compound can be added only in a limited amount and, therefore, retardation cannot be sufficiently controlled. Thus, it has been required to overcome this problem.